This invention relates to a controllable aqueous polishing technique for levelling the surface of films, particularly polycrystalline thin films and more particularly, semiconductors, and to products prepared therefrom.
As-grown semiconductor films such as, for example. CuInSe.sub.2 films, are particularly useful for producing p-n junction photovoltaic detectors, light-emitting diodes, heterojunction detectors, infrared detectors, solar cells and the like. A ternary chalcopyrite-type semiconductor such as CuInSe.sub.2 is particularly suited for use in photovoltaic devices such as single junction cells and its band gap is near optimum for tandem cells. Indeed, CuInSe.sub.2 has a direct band gap and one of the highest measured absorption coefficients in a semiconductor, minimizing the requirement for minority carrier diffusion length and related materials utilization. The electron affinity difference between CuInSe.sub.2 and CdS appears to be small, providing no interfacial conduction band spikes, and electrical properties can be varied widely both by doping and stoichiometry control. Solar cells prepared from CuInSe.sub.2 films are more particularly described by Birkmire et al. in High Efficiency CuInSe.sub.2 Based Heterojunction Solar Cells: Fabrication and Results, Solar Cells, 16, 419-427 (1986), incorporated herein by reference.
However, it has not been possible to grow a smooth film of a ternary chalcopyrite-type semiconductor such as CuInSe.sub.2 which has the required electro-optical properties for producing high conversion efficiency solar cells, particularly monolithic tandem cells. As-grown CuInSe.sub.2 films which have the necessary electro-optical properties for device applications, nominally those films about two to three microns in thickness, exhibit surface texture (growth facets and protrusions) in the 1 to 5 micron range. This was also observed by Arya et al during their investigation of CuInSe.sub.2 surface morphology and stoichiometry as reported in Photovoltaic and Structural Properties of CuInSe.sub.2 /Cds Solar Cells, Solar Energy Materials, 8, 471-481 (1983). The surface of CuInSe.sub.2 crystals contained so many microcracks and other non-uniformities that a device having an active area of 4.2 mm2 was reduced to 0.8 mm2 to eliminate regions with visible microcracks and low response.
In one application, CuInSe.sub.2 thin films are used in the production of monolithic tandem cells, wherein the CuInSe.sub.2 /CdS cell is combined with a wide bandgap cell such as CdTe/CdS or amorphous silicon. The CuInSe.sub.2 /CdS cell utilizes photons which are transmitted through the wide bandgap cell. In such a structure, the CuInSe.sub.2 /CdS cell serves as the substrate onto which the wide bandgap cell is fabricated. Because the second cell in a typical monolithic tandem cell construction (e.g. CuInSe.sub.2 /CdS/a-Si:H) mimes the surface texture of the base layer, the protrusions of the first layer are also present in the surface of the second. Accordingly, the very thin amorphous silicon top layer deposited on the second layer may be even thinner where it covers the protrusions in the surface below it. Consequently, such cells short on use and have low open circuit voltages. Thus protrusions, and irregularities in the as-grown surface result in product non-uniformity, low yield, and increased materials expense to insure adequate thickness in subsequently deposited layers to carry current and obviate breakdowns through shorting, particularly in tandem cells.
Several different kinds of surface treatment have been suggested for overcoming the problems posed by the rough surface of semiconductor films such as CuInSe.sub.2 films. Generally recommended surface treatments, including chemical etching, mechanical polishing, thermal oxidation, electrochemical oxidation, photoelectrochemical etching, air oxidation, and the like and their various recommended combinations, all have significant disadvantages, not least of which is the depletion of one or more film components, particularly in the near-surface area, and the degradation of physical and electro-optical properties.
Of the treatments suggested, the most practical surface modification method from a commercial standpoint for most applications would be chemical etching. Yakushev et al., The Observation of Near-Surface Deviations from Stoichiometry in CuInSe.sub.2 Crystals Following Chemical Etching, Solid State Communications, Vol. 65, No. 10, pp 1079-1083 (1988) studied the effect of chemical etchants employed for this purpose. They reported that significant near-surface composition changes which significantly affect the physical characteristics of the compound and the electro-optical properties of any device fabricated from it were observed with all etchants studied including 1:1 HCl:HNO.sub.3, 1:3 HF:HNO.sub.3, H.sub.2 SO.sub.4 :K.sub.2 Cr.sub.2 O.sub.7, and bromine in methanol. However, they noted that while very dilute solutions of potassium dichromate and sulfuric acid and very dilute solutions of bromine in methanol did not produce such significant changes, Cahen et al reported (J. Appl. Phys., 57, 4761 (1985)) that the use of bromine in methanol results in a copper-depleted surface and this result, which appears to be well known, was also reported by Arya et al.
As a practical matter, the use of a bromine/methanol etchant is further disadvantageous because the methanol tends to dissolve materials such as the resists used in lithographic and other semiconductor film processing techniques. This is particularly significant in the production of photovoltaic devices such as tandem cells, single junction cells, or any other devices which depend on smooth surfaced, uniformly layered films for operability. It has simply not been possible heretofore to remove large scale surface defects chemically and provide a specular finish on a semiconductor film such as CuInSe.sub.2 in a single step, without altering the stoichiometry of the film surface, and without degrading the electro-optical properties of the film and/or any device prepared therefrom.